1. Field of the Invention
The present invention relates to a semiconductor strain sensor having a polycrystalline silicon resistor used as a strain gauge.
2. Description of the Related Art
A semiconductor pressure sensor can detect a pressure with high accuracy using the variation in the resistance value caused by stress (piezoresistive effect) in a resistor that is formed by a semiconductor. Monocrystalline silicon has been conventionally used to form the semiconductor resistor. In recent years, however, development of a semiconductor pressure sensor using a polycrystalline silicon resistor has become significant. The polycrystalline silicon resistor can be easily formed on an insulator, and a high resistance substance can be formed by its thin film formation. Therefore a pressure measurement with high accuracy is expected over a wide temperature range. High resistance formation of the resistor also enables low power consumption in the pressure sensor.
In a semiconductor pressure sensor using the piezoresistive effect, in general, four resistors are formed on a pressure receiving member (diaphragm) and a full bridge is assembled. In this case, when monocrystalline silicon is used as a strain gauge, in order to avoid output fluctuation due to temperature, the semiconductor is formed so that the temperature coefficient of resistance (TCR) and an absolute value of a temperature coefficient of strain sensitivity (TCK) are equal but have opposite signs. That is, the sum becomes "0", and thereby self temperature compensation in constant current is performed. An impurity carrier concentration is adjusted in order to control the temperature coefficients.
The use of polycrystalline silicon resistors are described in the art. "POLYSILICON SOI PRESSURE SENSOR", in Sensors and Actuators, 17(1989)405-414, describes using a polysilicon resistor in a strain gauge environment and also describes a preferred impurity concentration range. "TEMPERATURE-INDEPENDENT PRESSURE SENSORS USING POLYCRYSTALLINE SILICON STRAIN GAUGES", published in Sensors and Actuators 17(1989) 521-527, describes using polysilicon resistors and maintaining the value TCR+TCK=0. Impurity carrier concentration dependence of the temperature coefficients is explained. In this case, the carrier concentration means that within crystal grains.
However, the film thickness of polysilicon shown in these references must be as thick as 0.4 .mu.m-0.5 .mu.m. Thin film formation is required for the high resistance formation and no way of accomplishing this is described by these references.